#### Black hole mergers and gravitational waves

**Black hole mergers and gravitational waves**

Existence of gravitational waves has recently been confirmed by the LIGO interferometer detection. The shape of the gravitational waveform dependence on time, found by the detector (after noise subtraction) is in a good agreement with that expected when the two black holes have just merged. The plot below presents the gravitational waveform computed from the simulation of the two merging black holes of mass ratio of q=0.85 and dimensionless spins of 0.2 i 0.3, for the more massive and lighter black hole, respectively. (Simulation with the code Einstein Toolkit. Author: Szymon Charzyński).

The animation below shows distribution of the real part of the Weyl scalar Psi4. Masses of the black holes simulated here were equal to 0.55 and 0.45 (in ADM dimensionless units), and their spins were 0.3 and 0.2. Curvature of spacetime is described by curvature tensor – the Riemman tensor (20 independent components), which can be decomposed into sum of Ricci and Weyl tensors (each with 10 independent components). For vacuum solutions of Einstein equations, the Ricci tensor vanishes, so curvature is described by Weyl tensor only. In the Newman-Penrose formalism components of Weyl tensor are encoded as five complex so called Weyl scalars Psi0, … , Psi4. These different components of curvature tensor have different physical interpretation, and Psi4 is considered to describe the outgoing radiation for asymptotically flat spacetime. See the movie with time visualisation below (author: Petra Sukova).

See the movie of the horizons of the two merging black holes below. The black hole mass ratio is 2, the less massive hole is spinless and the more massive has a dimensionless spin parameter of a=0.9. (author: Agnieszka Janiuk).

Next animation shows the apparent horizons location and the trajectories of two merging black holes during the inspiral (red and green), merger and ringdown (blue). The coordinates are noted in dimensionless units, and black holes are shown in in x-y plane. After merger, the product black hole drifts sideways, due to gravitational recoil effect. Assumed parameters: mass ratio of the holes q=3; their spins a1=0.9, a2=0. The black hole after the merger gained dimensionless spin a3=0.7. Click on the image below to see the movie (author: Szymon Charzyński).

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